1. Field of the Invention
This disclosure relates in general to magnetic read sensors for magnetic storage systems, and more particularly to a method and apparatus for an improved magnetic read sensor having synthetic or AP pinned layer with high resistance and high magnetoelastic anisotropy.
2. Description of Related Art
The heart of a computer is typically a magnetic disk drive which includes a rotating magnetic disk, a slider that has write and read heads, a suspension arm above the rotating disk and an actuator arm. The suspension arm biases the slider into contact with a parking ramp or the sheet of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing sheet (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the sheet of the rotating disk. When the slider rides on the air bearing the actuator arm swings the suspension arm to place the write and read heads over selected circular tracks on the rotating disk where field signals are written and read by the write and read heads. The write and read heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
Today's magnetic disk drives use sensors that are based upon giant magnetoresistance (GMR). In a GMR sensor, an external magnetic field causes a variation in the relative orientation of neighboring ferromagnetic layers. This in turn causes a change in the spin-dependent scattering of conduction. Thus the electrical resistance of the structure varies as the relative alignment of the magnetizations of the ferromagnetic layers changes.
One particularly useful application of GMR is a sandwich structure, called a spin valve. The spin valve includes two essentially uncoupled ferromagnetic layers separated by a nonmagnetic spacer metal layer, usually copper (Cu). In this structure the magnetization of one of the ferromagnetic layers, which is called pinned layer, is usually pinned by an adjacent antiferromagnetic layer (AFM layer), which exchange-couples to the pinned layer. The unpinned layer, or free ferromagnetic layer, is free to rotate in the presence of external magnetic field from the magnetic media. In spin valve structures the resistance between the free and pinned ferromagnetic layers is found to vary as a cosine of the angle between the magnetizations of these layers and is independent of the direction of current flow.
A variation of the sandwiched spin valve GMR structure comprises the synthetic or AP pinned spin valve structure, where the pinned layer is itself a sandwich structure comprising a first pinned layer (keeper layer) adjacent to the AFM pinning layer, and a second pinned layer (reference layer), that is naturally coupled in a direction opposite to the first pinned layer through an exchange coupling layer, that is usually ruthenium (Ru), sandwiched between the first and second pinning layer. The keeper layer may be referred to as AP1 and the reference layer as AP2. This structure may produce a low net magnetic moment to minimize the interaction between the free layer and the synthetic or AP pinned layer.
In general, the voltage output of the GMR spin valve sensor device is proportional to its sheet resistance, RS, the GMR ratio, ΔR/R, and average value of the cosine of the angle between magnetization of the free layer, θF, and magnetization of the pinned layer, θP, averaged over the stripe height. More specifically, the voltage output of the spin valve sensor device is given as: V=IS*(W/2H)*(RS*ΔR/R) <cos Δθ>.
Here <cos Δθ> is the average value of the cosine of the angle between magnetization of the free and pinned layers, averaged over the stripe height, and Δθ=θF−θP, IS the sensor current, W is the sensor track width, and H is the sensor throat height. In the ideal case, the pinned layer is fully saturated and its magnetization is perpendicular to the ABS everywhere in the stripe. However in reality this is not the case, because of several reasons which tend to keep pinned layer not fully saturated and rotate its magnetization away from ABS, thus reducing the average value of cos Δθ, and therefore reducing sensor signal output. One reason for that is a reduced exchange pinning due to, for example, patterning or lapping damage, especially near the ABS. Another reason is the magnetostatic energy of the pinned layer, which always tends to rotate pinned layer magnetization away from ABS and parallel to the stripe. These both reasons for reduced saturation magnetization of the pinned layer normal to ABS and a subsequent reduction in sensor amplitude become more important as the stripe height is shrinking. Therefore in order to improve signal output, it is important to have another mechanism to keep magnetization of the pinned layer normal to ABS. One way to do that is to induce strong magnetoelastic anisotropy perpendicular to ABS. The magnetoelastic anisotropy constant is given as Kσ=3/2 λPLσ. In case when total stress (σ) acting on the pinned layer is compressive (σ<0), the magnetostriction of the pinned layer, λPL must be positive in order to induce preferential anisotropy axis perpendicular to the ABS. Thus magnetoelastic anisotropy normal to ABS can be increased by increasing positive magnetostriction of the pinned layer, λPL. This anisotropy will provide strong pinning when interfacial AFM coupling between the antiferromagnetic layer and pinned layer is significantly weakened due to the reasons discussed above. When that happens, the magnetoelastic anisotropy pinning will take over the AFM pinning, and that will maintain high signal output.
As the recording density of disk drives increases, spin valve read heads are required to produce the same or larger signal output from smaller magnetic bits. As evident from the above discussion, this can be achieved by increasing, ΔR/R, Rs or λPL. Currently, pinned layers of a GMR read head are fabricated using binary alloys, such as CoFe (cobalt-iron). Using ternary CoFeX alloys can increase pinned layers sheet resistance, RS and magnetostriction, λPL without reducing its GMR ratio, ΔR/R, and thus increase spin valve head voltage output or amplitude.
Thus it can be seen that there is a need for a method and apparatus for an improved magnetic read sensor having synthetic or AP pinned layers with high resistance and high magnetoelastic anisotropy.